When applying non-expansive fire retardant coatings, substrate surface preparation is crucial for ensuring coating adhesion and fire resistance. The substrate material, surface condition, and environmental conditions directly affect the film-forming quality and durability of the coating. Therefore, comprehensive treatment from multiple dimensions, including cleanliness, smoothness, rust prevention, and environmental control, is necessary.
First, substrate surface cleanliness is fundamental for coating adhesion. Whether using steel or concrete substrates, oil, dust, rust, and loose materials must be thoroughly removed before application. Steel surfaces require sandblasting or mechanical grinding to Sa2.5 standard to ensure a uniform metallic luster and prevent residual rust or oxide layers from affecting the chemical bonding between the coating and the substrate. Concrete substrates require removal of surface laitance, oil stains, and loose particles. Cracks or holes must be repaired beforehand to prevent coating penetration and blistering. If an old coating exists on the substrate surface, its compatibility with the new coating must be tested. If incompatible, it must be completely removed and ground down to a solid base layer.
Secondly, the flatness of the substrate is crucial for coating uniformity. If there are protrusions or depressions on the steel structure surface, such as weld seams or bolt connections, they must be smoothed by grinding or filling to prevent excessive localized paint buildup that could lead to cracking. Concrete substrates require puttying or plastering to eliminate surface defects such as honeycomb and pitting, ensuring even paint coverage. For complex structures, such as steel beam joints or irregularly shaped components, a segmented treatment approach is necessary to ensure the flatness of each part meets requirements and prevent stress concentration caused by uneven coating thickness.
Rust prevention is the core aspect of steel structure substrate surface treatment. While non-expansive fire retardant coatings offer some thermal insulation, they cannot replace the function of rust-preventive primers. After cleaning, steel structures must be coated with a rust-preventive primer compatible with the flame-retardant paint to form a dense protective metal film, isolating oxygen, moisture, and corrosive media. The rust-preventive primer application must be uniform and without omissions, with particular attention paid to easily corroded areas such as weld seams and bolt holes. If the primer and flame-retardant coating are incompatible, it may lead to decreased adhesion between the coatings, or even trigger a chemical reaction that causes coating failure.
Substrate surface treatment must also consider the influence of environmental conditions. Before application, ensure the substrate temperature is at least 3°C above the dew point to prevent condensation from preventing the coating from drying and curing. The ambient temperature should ideally be controlled between 5°C and 35°C. Low temperatures will slow down the coating reaction, while high temperatures may accelerate solvent evaporation, causing blistering. Relative humidity should be controlled below 85%. A humid environment can prevent the moisture in the coating from evaporating completely, forming pores and reducing fire resistance. Furthermore, construction should be suspended during rainy or windy weather to prevent rainwater or dust from contaminating the uncured coating.
For special substrates or complex working conditions, surface treatment needs to be adjusted accordingly. For example, light metal substrates such as aluminum alloys require phosphating to enhance surface roughness and improve coating adhesion; substrates in high-temperature environments require high-temperature resistant anti-rust primers to prevent primer failure at high temperatures. If chemical contaminants are present on the substrate surface, they must be neutralized by solvent cleaning or alkaline washing to ensure surface neutralization. For structures with large dynamic loads, such as bridges or vibrating equipment, fiberglass mesh should be added to the coating to enhance crack resistance.
After surface treatment, a quality inspection is required. Inspection includes cleanliness, smoothness, integrity of the anti-rust layer, and environmental condition records. Cleanliness can be verified visually or by wiping with a white cloth; smoothness should be checked with a straightedge or laser rangefinder; and the anti-rust layer should be tested for adhesion using the cross-cut test or pull-out test. Only after passing the inspection can the next construction step proceed, ensuring coating quality is controlled from the source.
Substrate surface treatment is a fundamental step in the application of non-expansive fire retardant coatings, and its quality directly determines the fire resistance and service life of the coating. Strict cleaning, smoothing, rust prevention, and environmental control measures provide ideal adhesion conditions for the coating, thereby constructing an efficient and durable fire-resistant protective layer and providing reliable protection for building structural safety.
